It is used when talking about lining things up.

Some flash hiders are compensator as well and have solid prongs on the bottom. But it is only on the bottom if you get it lined up correctly.

Some muzzle devices are also better viewed some ways up, and they would need to be "timed" as well. Some screw on after market handguards need to be adjusted to fit the top rail as well otherwise they won't be properly vertical.

KBK